San Andreas. Not because of the size of the city, the atmosphere and the vibe it gave through Los Santos was amazing. RLS, The Ballas, Bicycles, Uzi's, The specific color of the sky and artists like Eazy E, Ice Cube, Dr Dre, Snoop Dogg, Cypress Hill, NWA...It just brang everything together to bring a West Coast experience..
Seriously though, San Andreas for me, I don't like the Mafia/Mob/Italian Mafia/Organized Crime themes to be honest, They don't really interest me. But in respect I do love Vice City and the depth of Liberty City for what it was back in it's time.